Gravitational waves have been detected for a second time

Gravitational waves emitted from the collision of two black holes have been identified by scientists for a second time.

The gravitational waves, which are ripples in the fabric of spacetime, were observed by researchers on December 26 using data collected from the Laser Interferometer Gravitational Wave Observatory (Ligo). The research team, involving more than 90 institutions around the world, has now confirmed the finding.

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The gravitational waves were sent travelling through spacetime when two black holes eight and 14 times the mass of our sun collided. The collision, which took place 1.4 billion years ago, created a more massive spinning black hole that is 21 times the mass of the sun. An additional sun's worth of mass was transformed into gravitational energy.

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The first detection, found in data collected on September 14, 2015, resulted from the merger of black holes 36 and 29 times the mass of our sun. Gabriela González, professor of physics and astronomy at the Louisiana State University and part of the Ligo team, said the latest detection was "very strong" despite the smaller sizes of the black holes.

"The nice thing about this detection is that because the black holes are smaller, they merge at a higher frequency," González told WIRED. "Our detected signal has a lot more cycles in the specific part of the Ligo detectors. For the first detection we could only see a few cycles."

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"It shows that black holes have diversity too," said González. "This is telling us there's a big spectrum of masses out there." The black holes colliding, as detailed in the journal Physical Review Letters journal, were smaller than the previous collision observed by scientists.

LIGO

The work, added González, was a starting point for being able to map the populations of black holes in the universe. The signals were produced from the final 27 orbits of the black holes before they merged. By looking at the arrival time of the waves' signals at both of the Ligo detectors it was possible for the scientists to roughly be able to position the source in the sky.

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"In the near future, Virgo, the European interferometer, will join a growing network of gravitational wave detectors, which work together with ground-based telescopes that follow-up on the signals," Fulvio Ricci from Virgo said in a statement. "The three interferometers together will permit a far better localisation in the sky of the signals."

Albert Einstein first predicted gravitational waves in 1916, following his theory of general relativity. He said gravitational waves were ripples in the curvature of spacetime that travel outward from the source that created them (in this case the merging of the black holes). He argued that spacetime – which is any mathematical model that combines space and time – would create ripples that move across the universe at the speed of light.

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When the gravitational waves were discovered for the first time the Ligo scientists were able to listen to the waves by converting the distortions into sound. This time, they heard a series of 'chips' as the recorded frequency was shifted by the waves. "It was different in a sense that it was not as loud," said González. "With the first one we were very surprised that you could clearly see it in the data."

The second detection of the waves also makes it more likely that scientists looking at Ligo data will be able to forecast waves discovered in the future. The machine will be turned back on for six months later this year after its detection sensitivities have been improved.

The improvements of the machines will mean that Ligo is able to reach between 1.5 and two times more of the volume of the universe than it was previously able to.